Method of manufacturing an electrolytic grinding wheel



May 12, 1970 7 METHOD OF MANUFACTURING AN ELECTROLYTIC GRINDING WHEEL Filed May 18, 1967 1 FIG. 2

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WET ABRASWE FORM ovERALLMw 0F WET ABRASWE W\TH BLEN D OFCoN DUCTWE POWDER AND \NoReAmc BoNDlNE MATERW.

POUR OVERALL M\X lNTO MOLD SUBJECT MOLD TO PRESSURE DRY MOLDED UN\T ELEVATE TEMPERATURE OF MOLDED UNIT TO MATURWG LEVEL OF BOND CoNDucTlVE POWDER BLEND FINEL'Y DwmED CONDUCTNE POWDER WlTH AN \NORGAN\C BONDING MATER\A\ WET ABRASIVE PARTICLES AND COMBI NE wn'u THE BLEND 0F BOND\NG MATERIAL AND CONDUCTWE POWDER TO FORM AN OVER ALL MW Mom CoNDucTwE UNLT FROM ovEEALL M\ CONDUCTWE. POWDER HRE MOLDED UN\T AY A TEMPERATU RF. BELow THE MELTING P0\NT OF THE JOHN N RIXSEO RL NALD J. G ERRY ATTORNEY United States Patent 3,510,994 METHOD OF MANUFACTURING AN ELECTROLYTIC GRINDING WHEEL John J. Amero, Shrewsbury, and Ronald J. Gerry, .Worcester, Mass., assignors to Norton Company, Worcester, Mass., a corporation of Massachusetts Filed May 18, 1967, Ser. No. 639,326 Int. Cl. B24d 3/34 US. Cl. 51--295 4 Claims ABSTRACT OF THE DISCLOSURE A method for making conductive grinding wheels for electrochemical grinding is disclosed. The wheel comprises abrasive grains bonded by a ceramic material having a maturing temperature below the melting temperature of a conductive metal powder incorporated in the mix used to form the Wheel. A glass bond including lead oxide and boron oxide having a maturing temperature of 600 to 800 C. is disclosed. Silver is disclosed as a conductive metal, and from 4 to 9% silver content is suggested in the finished grinding tool.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a grinding wheel for electrochemical grinding and method for making ceramic bonded grinding wheels for use in electrochemical grinding.

Description of prior art Because a vitrified wheel is of ceramic composition, it is fired at relatively high temperatures and is diificult to render conductive. One technique is to impregnate the wheel with a conductive substance. This requires an extensive number of secondary steps, after the initial manufacture of the wheel, such as drying and firing. In addition, the impregnation which coats the walls of the pores in the wheel tends to be non-uniform, giving rise to variations in performance from one wheel to another. The impregnation also tends to be non-uniform within an individual wheel with resulting variations throughout the wheel in internal conductivity.

In electrochemical grinding, resinoid wheels tend to hold their form longer than their vitrified counterparts, but the latter are easier to shape. Special shapes requiring the removal of a significant amount of stock after molding are much easier to prepare as vitrified products.

SUMMARY OF THE INVENTION An object of this invention is to achieve a vitrified grinding wheel which is conductive without having been impregnated by conductive substances. A related object is to produce a vitrified, conductive grinding wheel by a unitary manufacturing process which does not require secondary steps after initial manufacture. Another related object is to enhance the uniformity of the characteristics of vitrified conductive grinding wheels from one wheel to another by producing a wheel in which the conductive agent is within the bond rather than coating the pores.

Another object of the invention is to increase the efficiency of electrochemical grinding, particularly when using vitrified grinding wheels.

In accomplishing the foregoing and related objects, the invention provides for the formation of a conductive grinding body from abrasive particles mixed with a ceramic bonding material containing a conductive powder. The ceramic bond is formed below the melting point of the conductive powder, which is distributed as interconnected agglomerates of fine metallic particles within the bond itself.

In a preferred embodiment of the invention, the conductive powder takes the form of finely divided silver particles and the ceramic bonding material is primarily a composition of metallic oxides and clay. The metallic oxides are collectively of a composition that fuses at a temperature belowthe melting point of the silver powder. As a result, a grinding unit of suitable conductivity is realized in which the conductive metal is distributed throughout the grinding unit in conductive association. By contrast, fusion of the bonding material at a temperature above the melting point of the conductive powder would result in the substantial formation of droplets which would be relatively ineffective in producing the desired conductivity within the grinding unit.

To fabricate a conductive grinding body in accordance with the invention, finely divided conductive powder is mixed with a ceramic bonding material. Abrasive particles are next wetted and combined with a blend of the bonding material and conductive powder in order to form an overall mix. The overall mix is then molded into suitable form, such as a grinding wheel. Once molded, the grinding unit is fired at a temperature below the melting point of the conductive powder.

In the preferred embodiment of the invention, finely divided silver powder is intimately mixed with an inorganic bonding material constituted primarily of clay and glass based on lead oxide. The mix is then molded and fired at a temperature below the melting point of the silver powder.

When incorporated into a grinding system, a conductive grinding wheel in accordance with the invention is driven by a motor and employed in conjunction with a source of electrical energy which establishes a partial circuit between the grinding wheel and stock being worked. The circuit is completed by introducing an electrolyte between the face of the grinding wheel and the stock. Surface pores in the grinding wheel are occupied by the electrolyte which acts with conductive powder distributed within the grinding wheel to facilitate electrolyticallyassisted grinding.

It is theorized that the porosity of a vitrified wheel promotes better distribution of the electrolyte applied thereto and, acting in conjunction with conductive metal distributed within the wheel, permits greater size control over the stock or workpiece than can be achieved with a comparable grinding wheel of resinoid bond. Moreover, because the bond is vitreous, the bonding material flows during the firing of the wheel to bring about a natural interconnection of the conductive metal particles. As a result, there appears to be a further enhancement in the electrochemical grinding performance of the product.

Other aspects of the invention will become apparent after considering an illustrative embodiment and fabrication process thereof, taken in conjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a flow chart summarizing the operations employed in fabricating a conductive grinding wheel in accordance with the invention; and

FIG. 2 is a flow chart detailing the operations summarized by the flow chart of FIG. 1.

In fabricating the wheel, the mix is formed by combining water wetted abrasive with a blend of an inorganic bonding material and finely divided conductive powder. The resulting mix is deposited in a mold of appropriate dimensions.

DESCRIPTION OF PREFERRED EMBODIMENTS In a preferred embodiment of the invention, the finely conductive powder is of silver. In a tested model of the invention the silver powder consisted of individual particles of average size of about one half to one micron and present to the extent of approximately 8 percent by volume of the overall mix for an abrasive size of approximately 80 grit. Grit size refers to selecting the size of the grain as specified in Simplified Practice Recommendation 118-50 of the US. Department of Commerce.

In general the appropriate amount of silver depends upon grit size; as the grit size is reduced, the silver content is increased, since there is a greater total area of abrasive tobe coated by the powder. With relatively fine abrasive of 120 grit, 9 volume percent silver was found satisfactory. For 100 grit wheels, 4.8% of silver by volume produced an essentially non-conductive wheel unsuitable for electrochemical grinding, while 6% produced a highly conductive wheel suitable for electrochemical grinding. With 46 grit abrasive, only 4% silver was required, while with only 2% silver, the 46 grit wheel was non-conductive. The volume percent is based on the total volume of abrasive, bond (including the silver) and pores.

Silver is the preferred metal to use because it is not subject to deleterious oxidation in the firing of the wheels. Other metals such as platinum are useful but expensive. Copper, unless the particles are protected against oxidation by a silver or other coating, is not desirable. Reducing or nitrogen atmospheres may be deleterious to the ceramic bond, and thus not recommended in most cases even though their use would avoid the oxidation problem in the firing of the wheel. Although the percentage of silver content can be made higher than 9%, there is little to be gained after the necessary level of conductivity has been reached, except for special applications where very high conductivity may be desirable. In such cases, the upper limit of the silver content is restricted only by the physical properties of the fired wheel.

The abrasive may be any of the materials commonly used in the manufacture of grinding wheels, such as silicon carbide. In a tested model of the invention the abrasive was fused alumina and amounted to 48 percent by volume of the overall mix.

The remaining item of the overall mix is the inorganic bonding material. In the case of vitreous bonds, the bonding material is of glass-like composition which fuses at a temperature below the melting point of the conductive metal powder. Useful bonds are those that mature at from 600 to 800 C. Such a composition is principally of clay and glass. The glass is desirably in a form known as frit, which is so called because it is produced by the melting and rapid quenching of oxide materials to produce small particles. The principal ingredients of the clay are hydrous aluminous materials, including aluminum silicate.

Besides glass frit and clay, the bonding material may include such substances as sodium carbonate, i.e. soda ash, and boric acid. In a tested model of the invention, the bonding material was 50 percent by weight glass frit; 35 percent by weight plastic clay, commonly designated as Imperial ball clay; 7 percent by weight soda ash and 8 percent by weight boric acid.

In the same tested model of the invention the glass frit, the major ingredient of the bonding material, was in turn formed by mixing a number of oxides, the controlling consideration being that the melting point of the resulting composition be below that of silver powder.

For a tested model employing a glass frit based on lead oxide, the latter was present to the extent of 59.2 percent by weight. Other ingredients of the frit were silicon dioxide, 20 percent by weight; boric oxide, 14.4 percent by weight; and sodium oxide, 6.4 percent by weight.

The process of preparing conductive grinding media in accordance with the invention is summarized by the flow chart of FIG. 1. Initially, finely divided conductive powder is intimately blended with an inorganic bonding material of composition and characteristics similar to those described previously. Next, wetted" abrasive" particles are combined with the blend of the conductive ppw; der and bonding material to form an overall mix. The overall mix is poured into atmold and pressed, following which firing takes place at a temperature below the melting point of the conductive powder.

Details of the process summarized by the flow chart of FIG. 1 are set forth in the flow chart of FIG.;2,.se t ting forth the steps used in fabricating a'test model of the invention.

In accordance with the first step of the flow chart of FIG. 2, finely divided silver powder is obtained. Such metal powder is obtainable commerciallydown to an average particle size of approximately one half to one micron and was used in a tested model of the invention. Alternatively, suitably finely divided conductive metal powder may be obtained in standard fashionby precipitation.

In the next step of processing, the conductive powder is coarse-mixed with an inorganic bonding material having the characteristics described previously. The conductive powder and bonding material are then intimately blended. In a tested modelof the invention, silver powder was intimately blended with a bonding material, primarily of ball clay and lead based glass, by being passed twice through a mesh screen.

In the fourth step of processing, the abrasive is mixed with Water until uniformly wet. A test model of the invention added abrasive in the form ofv fused alumina to a mixer to which water was added until the alumina was uniformly wet.

As mixing continues, the bond-conductive-power blend is added slowly to the wetted abrasive and. mixed until the abrasive appears to be uniformly coated.

For a tested model of the invention, the mix amounted to 48 percent by volume abrasive, 10 percent by volume bonding material, 8 percent by volume silver and the remainder water. In one test the actual quantities were 600 grams abrasive, 113 grams bonding material with a base of lead oxide, 268.2 grams of fine silver powder, and 19.5 cubic centimeters of water.

Once the overall mix is prepared, it is poured into a mold. For a test model of the invention, the mold consisted of conventional constituents, including a mold band, arbor, top plate and bottom plate. The mold pressure was approximately 3 tons per square inch.

After molding is completed, the wheel is dried and fired in air in order to properly fuse the bonding material. In a test model of the invention, the molded wheel was raised from room temperature to a firing temperature of 800 C. at a rate of 100 C. per hour and, once the firing temperature of 800 C. was attained, it was maintained for two hours. It is to be noted that the firing temperature of 800 C. is below the melting point of conductive silver powder and, at the same time, well below the typical firing temperature of 1200 C. for vitrified bonded wheels. The latter temperature, being above the melting point of silver, would result inthe formation of a large number of droplets of metalic silver, breaking up the network of conductive paths, and thus materially reducing the suitability of the wheel for electro-chemical grinding.

After being fired to mature the bond, test "grinding wheels, manufactured in accordance with the invention, were sided, faced and reamed in conventional fashion to cylindrical disks A2 inch in thickness and 7 inches in diameter. Each disk was provided with a hole 1% inches in diameter for mounting on a spindle of a conventional electrolytic grinding apparatus. The test wheels were'inspected and speed tested to verify their suitabilityi for use in commercial grinding operations.

To measure the performance of the test wheelsthey were run on a commercially available .6 inch x 18 inch electrolytic type surface grinder. I t

Where the stock or workpiece was of die steel, marketed and sold under the trade name Huron and the electrolyte was of a type marketed and sold under the trade name Anocut E8, the depth of cut was 0.010 inch for a traverse rate of 1.5 inches per minute, resulting in a spindle power of 75 watts for a current draw of 200 amperes. Similar high performance results were obtained for all test grinding wheels manufactured in accordance with the invention.

By contrast, impregnated vitrified grinding wheels, operating under similar conditions, were found to vary widely in their performance, the most favorable test giving a spindle power of 975 watts for a current draw of 95 amperes. Since the current draw reflects the electrolytic work done, and the spindle power the mechanical work, it is seen that vitrified grinding wheels in accordance with the invention are more efficient than the impregnated wheels.

Other adaptations and modifications of the invention will occur to those skilled in the art.

We claim:

1. The method of manufacturing a conductive grinding tool comprising the steps of:

(1) mixing finely divided conductive powder selected from the group consisting of silver, platinum, and silver coated copper with glass bonding material;

(2) combining the the abrasive with the mixture of said bonding material and said conductive powder to form an overall mix which, as fired, will contain from 4 to 9% by volume of conductive metal;

(3) molding said overall mix to shape; and

(4) firing said shape at a temperature below the melting point of said conductive powder.

2. The method as defined in claim 1 wherein said finely divided conductive powder is of silver.

3. The method as defined in claim 1 wherein said bonding material comprises glass frit, clay, boric acid, and sodium carbonate.

4. The method as defined in claim 1 wherein said conductive powder is silver and said firing temperature is below 800 C.

References Cited UNITED STATES PATENTS 2,233,176 2/1941 Melton et a1. 5l298 2,555,174 5/1951 Whittaker et a1. 51-308 2,578,167 12/1951 Bjorklund 5 1309 3,062,633 11/1962 Coes 51295 3,203,775 8/1965 Cantrell 51-307 3,216,854 11/1965 Halverstadt 5 l-295 3,283,448 11/1966 Thompson 5l298 3,433,730 3/1969 Kennedy 5l298 DONALD J. ARNOLD, Primary Examiner US. 01. X.R. 51-408 309 

